Change the play list back end to cache recently used records rather
than always rereading data from the data base.

Implement play_list_save_m3u (in src/playlist.c).

Enable basic filtering by e.g. artist, album, search terms, etc.

Add a play queue: drag elements to play queue to queue them.

Enable multi-select and allow dragging selected items to move them.

When importing, don't import the same file again (i.e., detect
duplicates).

(Re-)enable saving play lists. Cf. src/main.c and src/playlist.c.

Try harder to not freeze the interface on long imports (it is very
unresponsive--especially at the start).

If we do a big import and then quit before all the meta data is read,
it will only be read once we play the song.  It would be nice to
search the data base for entries which are not yet initialized as well
as those which are out of date.

Maintain a list of directories which have been added and then scan for
new files.
